The present invention relates to voltage controlled oscillators (VCO), and more particularly, those that are used in television recorders.
Voltage control oscillators are oscillators where the generated frequency is controlled by a voltage applied to the oscillator. Therefore, these oscillators are useful in applications where a reference frequency signal and the voltage control oscillator output signal have to be phase locked together. If one needs a signal at a frequency which is a multiple of a reference frequency, it is typical to use a voltage control oscillator to generate the signal, apply a sample of its output signal to a divide-by-N counter or divider, and then compare the output signal of the divider with the reference signal, so that the VCO output signal frequency is N times as high as the reference frequency. These kind of oscillators are used, for instance, in video tape machines, especially for the so-called "Gen-Lock" module used during recording, which module has an internal frequency generator, so that the machine is running very close to standard recording speed when no video reference signal is coming in. On the other hand, the machine has to lock to the incoming reference signal almost immediately when the reference signal is available. Because the tolerances of the incoming video signals are standardized and must not exceed the tolerances which are laid down in the SMPTE Rules and Recommendations, one would like to have a voltage control oscillator which keeps its frequency within this range, even if there is no incoming reference signal. This requires an oscillator frequency control voltage when no reference signal is present which is as close as possible to the control voltage when an incoming signal is present.
It is therefore desirable to have a control voltage for the voltage control oscillator which has a value which is as nearly the same as possible with and without incoming signals.